What is the best high way gear for a ford 9 inch? I'm running a 355 with a 350 turbo transmission?

What is the best high way gear for a ford 9 inch? I'm running a 355 with a 350 turbo transmission?

You need to consider the transmission gear ratios as well as the rear axle gears to determine the best gearing for your car. MOST modern transmissions have an overdrive top gear, which means it's OK to run a lower (higher numerically) rear axle ratio for better acceleration in the lower gears. The BEST gearing should give you an engine RPM in the middle of the power band while running at highway speeds in top gear.

Something around 3:08 would be good highway gearing, but if it matters, your acceleration would suffer. You would still be able to get on the freeway OK, but you wouldn't win any drag races unless you have one of those Hellcat engines. LOL

My Tundra has a 4:10 rear gear ratio, but 5th gear in the trans is Overdrive, and 6th gear is MORE overdrive, so, as JetDoc says, the lower rear gear is no problem.

You COULD swap in T700R4 trans, which has overdrive 4th gear.

3.00 is a good road gear and its commonly available for the 9"

Since the TH350 is a 3 speed, Something like a 3.00 or 3.25 would be a appropriate. I wouldn't go beyond that without OD though.